Add comments to next LiteCommand (#2846)

This commit applied adds comments preceding a command to the LiteCommands new
field `comments`.

This can be usefull for example when defining a function with `def`. Nushell
could pick up the comments and display them when the user types `help my_def_func`.

Example
```shell
def my_echo [arg] { echo $arg }
```
The LiteCommand def will now contain the comments `My echo` and `It's much
better :)`.

The comment is not associated with the next command if there is a (or multiple) newline
between them.
Example
```shell

echo 42
```

This new functionality is similar to DocStrings. One might introduce a special
notation for such DocStrings, so that the parser can differentiate better
between discardable comments and usefull documentation.

crates/nu-cli/src/commands/command.rs

@@ -250,7 +250,7 @@ impl WholeStreamCommand for Block {
     }
 
     fn usage(&self) -> &str {
-        ""
+        &self.params.usage
     }
 
     async fn run(&self, args: CommandArgs) -> Result<OutputStream, ShellError> {